This type of procedure and this type of device for creating a optical output signal are already known, for example through the publication of J. J. O'Reilly et al., "RACE R2005: micro-wave optical duplex antenna link", IEB Proceedings--J, Vol. 140, No. 6, December 1993, pages 385-391.
The optical transmission device shown there in FIG. 2 (dual frequency optical source) comprises a DFB laser, a primary optical modulator, a signal source and an optical filter. The light emitted by the DFB laser is a continuous signal (continuous wave signal) with an optical frequency .nu..sub.0 (carrier frequency). This light is fed to an input on the primary optical modulator which is controlled by the signal source. The light emerging from the optical modulator has a frequency spectrum with two impressed frequency parts (sideband carriers) which are arranged symmetrically, at the spacing of the modulation frequency F.sub.m of the signal source, around the suppressed optical carrier frequency .nu..sub.0, i.e. both frequency parts have a frequency distance of 2 F.sub.m. The optical filter switched after the output of the primary optical modulator separates both frequency parts which are respectively applied to an output on the optical filter.
In order to transmit light modulated by a data or message signal using such an optical transmission device, one (the first) output of the optical filter is connected to a second optical modulator and the other (second) output using a coupler. Using the data signal, the second optical modulator modulates the light part emerging at the first output of the optical filter. This is an external modulation of the light parts emerging at the output of the optical filter. The frequency spectrum of the modulated light part therefore has only one of the two frequency parts. This modulated light part which emerges at the output of the second optical modulator, and the light part emerging from the second output of the optical filter are launched through the coupler as an optical output signal in a fibre optic. The launched light is therefore made from two light parts: a light part modulated using the data signal along with one sideband carrier and an unmodulated light part with the second sideband carrier. This combined light lands in an optical receiver on a photodiode which produces the desired data signal in the millimeter (mm) wave range through coherent superimposition of both light parts.
In this known device, both light parts and the modulation of one of the two light parts is carried out in separate devices. Therefore this known device is more expensive to manufacture.